In most sets, casting something that’s 9+ is insane. You will be long dead before you get that close.
Zendikar is not most worlds.
If you want to cast one of these behemoths, you’re going to need to produce lots of mana.
There a a few different wants to do this:
Defenders are based in GW with aggressive options in Red.
Because mana is useful for so many different archetypes, if you want to be casting eldrazi, take ramp/spawns over practically everything except possibly Kozilek / Ulamog.
Even with massive ramp, Emrakul is still near impossible to get out.
What else can you do with eldrazi spawn?
Most of the set is all about going tall (one giant creature) but there are several options for going wide (lots of little creatures).
Broodwarden turns all your spawn into bears. (2/2).
costs 8, but once he’s in play you can turn your spawns into 7/7 for (1) each.